cmd/compile: recognize OpS390XLoweredNilCheck as a nil check in the scheduler

Before this change a nil check on s390x could be scheduled after the
target pointer has been dereferenced.

Change-Id: I7ea40a4b52f975739f6db183a2794be4981c4e3d
Reviewed-on: https://go-review.googlesource.com/29730
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/schedule.go

@@ -97,7 +97,10 @@ func schedule(f *Func) {
 					f.Fatalf("LoweredGetClosurePtr appeared outside of entry block, b=%s", b.String())
 				}
 				score[v.ID] = ScorePhi
-			case v.Op == OpAMD64LoweredNilCheck || v.Op == OpPPC64LoweredNilCheck || v.Op == OpARMLoweredNilCheck || v.Op == OpARM64LoweredNilCheck || v.Op == Op386LoweredNilCheck || v.Op == OpMIPS64LoweredNilCheck:
+			case v.Op == OpAMD64LoweredNilCheck || v.Op == OpPPC64LoweredNilCheck ||
+				v.Op == OpARMLoweredNilCheck || v.Op == OpARM64LoweredNilCheck ||
+				v.Op == Op386LoweredNilCheck || v.Op == OpMIPS64LoweredNilCheck ||
+				v.Op == OpS390XLoweredNilCheck:
 				// Nil checks must come before loads from the same address.
 				score[v.ID] = ScoreNilCheck
 			case v.Op == OpPhi: